Document requirement for ZStandard compression

What does this MR do and why?

Document requirement to support ZStandard compression for Debian binary packages.

Since dpkg 1.21.18 compression zstd is supported. It is used by default since Ubuntu 21.10. The Gitlab Debian package registry will not recognize such binary packages as valid and fails with

Invalid Package: failed metadata extraction

Document requirement to support zstd.

#422584

Screenshots or screen recordings

How to set up and validate locally

• Enable Debian API for this project by an Administrator until #337288 is resolved.
• See sample project https://gitlab.com/pmhahn/debian-package-registry

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

doc/api/packages/debian_group_distributions.md

@@ -83,7 +83,7 @@ GET /groups/:id/-/debian_distributions/:codename
 | Attribute  | Type           | Required | Description |
 | ---------- | -------------- | -------- | ----------- |
 | `id`       | integer/string | yes      | The ID or [URL-encoded path of the group](../rest/index.md#namespaced-path-encoding) owned by the authenticated user. |
-| `codename` | integer        | yes      | The `codename` of a distribution. |
+| `codename` | string         | yes      | The `codename` of a distribution. |
 
 ```shell
 curl --header "PRIVATE-TOKEN: <your_access_token>" "https://gitlab.example.com/api/v4/groups/5/-/debian_distributions/unstable"
@@ -122,7 +122,7 @@ GET /groups/:id/-/debian_distributions/:codename/key.asc
 | Attribute  | Type           | Required | Description |
 | ---------- | -------------- | -------- | ----------- |
 | `id`       | integer/string | yes      | The ID or [URL-encoded path of the group](../rest/index.md#namespaced-path-encoding) owned by the authenticated user. |
-| `codename` | integer        | yes      | The `codename` of a distribution. |
+| `codename` | string         | yes      | The `codename` of a distribution. |
 
 ```shell
 curl --header "PRIVATE-TOKEN: <your_access_token>" "https://gitlab.example.com/api/v4/groups/5/-/debian_distributions/unstable/key.asc"
@@ -253,7 +253,7 @@ DELETE /groups/:id/-/debian_distributions/:codename
 | Attribute  | Type           | Required | Description |
 | ---------- | -------------- | -------- | ----------- |
 | `id`       | integer/string | yes      | The ID or [URL-encoded path of the group](../rest/index.md#namespaced-path-encoding) owned by the authenticated user. |
-| `codename` | integer        | yes      | The codename of the Debian distribution. |
+| `codename` | string         | yes      | The codename of the Debian distribution. |
 
 ```shell
 curl --request DELETE --header "PRIVATE-TOKEN: <your_access_token>" "https://gitlab.example.com/api/v4/groups/5/-/debian_distributions/unstable"